Non-volatile memory cells, such as ferroelectric memory cells, are used in semiconductor memory devices. Such ferroelectric memory cells employ the use of ferroelectric metal oxide ceramic materials, such as lead zirconate titanate (PZT). Other types of ferroelectric material, such as Strontium-bismuth-tantalate (SBT) or lead-lanthanum-zirconium-titanate (PLZT) may also be used. FIG. 1 shows a ferroelectric memory cell 105 having a transistor 130 and a ferroelectric capacitor 140. A capacitor electrode 142 is coupled to a plateline 170 and another capacitor electrode 141 is coupled to the transistor which selectively couples or decouples the capacitor from a bitline 160, depending on the state (active or inactive) of a wordline 150 coupled to the transistor gate. A plurality of memory cells are interconnected by wordlines, bitlines, and plateline(s). Support circuitry is provided to facilitate access to the memory cells.
The ferroelectric memory stores information in the capacitor as remanent polarization. The logic value stored in the memory cell depends on the polarization of the ferroelectric capacitor. To change the polarization of the capacitor, a voltage which is greater than the switching voltage (coercive voltage) needs to be applied across its electrodes. An advantage of the ferroelectric capacitor is that it retains its polarization state after power is removed, resulting in a non-volatile memory cell.
The memory cells can be accessed by, for example, a processor to retrieve information (e.g., data word). The width of the data word depends on the memory architecture. For example, the width of the word can equal to 4 bits. Other data word lengths can also be useful. When a data word is accessed, errors may occur. For example, 1 bit of the data word can be erroneous due to a defective memory cell. To avoid or reduce the probability of a system failure due to defective memory cells, error correction codes (ECC) or techniques are employed. ECC techniques detect and correct, if possible, errors in a data word. The number of erroneous bits in a data word which can be corrected depends on the error correction technique employed.
As can be seen, ECC can enhance the operational reliability by correcting errors in a data word. However, in the event that the number of errors in a data word exceed the capabilities of a particular ECC, a system failure would occur. For example, for ECC capable of repairing one bit, a second failing bit within the accessed data word would lead to a total failure.
From the foregoing discussion, it is desirable to avoid or reduce failures caused by memory errors in ICs.